1. Field of the Present Invention
The invention relates to a wireless communication system including a transmitting device having a plurality of antennas and capable of transmitting different radio signals from the respective antennas, and to the transmitting device.
2. Description of the Related Art
MIMO (Multi Input and Multi Output) communication system is given as a communication system capable of improving a transmission rate (transmission capacity) as a total by transmitting different pieces of data by use of same frequency band (and further the same spread code) from a plurality of antennas in parallel. The MIMO communication system is that plural pieces of data are transmitted from a plurality of transmitting antennas in parallel, and the signals synthesized while passing through a variety of communication paths are received by a plurality of receiving antennas. FIG. 7 is a diagram showing an outline of the MIMO communication system. FIG. 7 shows, in the MIMO communication system configured by i-pieces of transmitting antennas 500 and j-pieces of receiving antennas 510, how plural pieces of data (x1-xi) are transmitted to the receiving antennas 510 from the transmitting antennas 500, and the respective antennas obtain signals y1-yj synthesized with these pieces of data (x1-xi).
Note that when the transmission signal from each antenna is designated by a transmission vector x, the receipt signal received by each antenna is designated by a receipt vector y, a state of a radio path is expressed as a channel matrix H, and a noise vector is designated by n, there is established a relationship such as y=Hx+n.
In the MIMO communications as shown in FIG. 7, a receiving-side device receiving the signals transmitted from the plurality of antennas and then synthesized, utilizes a method called MLD (Maximum Likelihood Detection) defined as a maximum likelihood decoding method in order to acquire an excellent radio characteristic. By this method, the receiving-side device detects a necessary piece of data by separating the synthesized signals. The MLD is a method of detecting a data pattern by judging, with respect to combinations of all the transmission data patterns that can be transmitted by the transmitting side, if transmitted in such a manner, how much a possibly-acquired receipt signal gets approximate to the actual receipt signal (a degree of maximum likelihood) (see FIG. 8). In the MLD, however, in the case of transmitting the signals from, for example, four pieces of transmitting antennas by 16 QAM (Quadrature Amplitude Modulation) defined as a digital modulation method of transmitting 4-bit data with one symbol, there is a necessity of obtaining the likelihood of data patterns numbered as tremendously as 65536 (=164). In this case, it follows that the receiving-side device detects the data pattern exhibiting the maximum likelihood from within this tremendous number of data patterns. Thus, the MIMO communication system requires an enormous throughput for the data detection.
A method for solving this problem involves employing Pre-Rake, etc. shown in FIG. 9(B) in the transmitting-side device. The method typified by Pre-Rake is a method for reducing the receiving-side processes by the signal processing on the transmitting side. For instance, FIG. 9 shows wireless communications based on normal CDMA (Code Division Multiple Access) (FIG. 9(A)) and CDMA-based wireless communications using Pre-Rake (FIG. 9(B)). In the normal CDMA-based wireless communications shown in FIG. 9(A), the receiving side detects the data by the signal processing (channel compensation) based on the transmission path information. On the other hand, in the case of employing Pre-Rake shown in FIG. 9(B), the signal processing is previously executed based on the transmission path information of the signal before transmitting the signals.
In the Pre-Rake method, for example, in the case of a transmission environment (an environment where a path 1 and a path 2 shown in FIG. 10(A) exit) as shown in FIG. 10(A), though normally the receiving side makes channel compensation corresponding to the transmission environment, the transmitting side executes a channel compensation process equivalent to that on the receiving side.
For instance, a weighting synthesis (Rake creating) unit as shown in FIG. 10(B) multiplies the transmission signal by a weighting coefficient of each transmission environment. With this operation, the receiving-side signal processing can be reduced.
A technology disclosed in the document (“Examinations about Configuration of Transmitter/Receiver of MTMT Array System Using Weight Batchwise Control at Base Station”, written by Hoshida, B-5-54, General Meeting of Electronic Information Communication Institution in 2002) is proposed as a method of increasing a channel capacity by executing this type of signal processing employing the transmission path information on the transmitting side in the MIMO communication system.
That is, in the MIMO communication system, there are proposed a method of using an MLD receiver requiring an enormous throughput for acquiring an excellent radio characteristic and a method of employing a simple receiver requiring merely a low throughput by executing the signal processing that previously takes account of the transmission path on the transmitting side.
A base station performing the MIMO communications, however, has a case of desiring to separately use the MLD receiver and the simple receiver. In this case, there is none of a method of making the above methods coexistent with each other.